In electrodes for secondary batteries such as lithium-ion batteries, an active material is bound by a binder to a current collector of each of a positive electrode and a negative electrode. As the active material for the positive electrode of the lithium-ion batteries, transition metal oxides such as LiCoO2 have been conventionally used. In recent years, from the viewpoints of economics and resource reserves, application of polyanion materials such as LiFePO4 composed of lithium, a transition metal, and a phosphoric acid anion has also been taken into consideration. Further, from the viewpoint of low costs and availability of elemental components, active materials especially composed of LiFePO4 is under development.
On the other hand, oxidation resistance is required for a binder for a positive electrode, and thus, conventionally, fluorine binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) have been frequently used. Further, for a binder for a negative electrode, polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) have been frequently used.
Here, the fluorine binders have high oxidation resistance, but have low adherence to an active material and a current collector. Thus, a large amount of binder is necessary for binding the active material and the current collector together. However, when a large amount of binder is contained in a mixture layer, there is a problem such that the active material is more likely to be coated with the binder, thus causing the lowering of battery properties.
On the other hand, the above-mentioned SBR has high adherence to the active material and the current collector. Thus, even when there is less amount of binder, the active material is sufficiently bound. However, since affinity between SBR and the active material is too high, there is a problem such that the surface of the active material is more likely to be coated with SBR. Further, PVDF and SBR have high affinity to an electrolyte of a secondary battery. Thus, when a secondary battery is left standing at a high temperature or subjected to repetitive charging and discharging, there has also been a problem of battery expansion caused by swelling of the resin.
In order to solve such problems, it has been considered to employ, as a binder, an olefin copolymer which is electrochemically stable and which is less likely to swell even when immersed in an electrolyte (PTLs 1 and 2). An olefin copolymer binder is excellent in resistance to oxidation-reduction, undergoes less swelling by an electrolyte, and is less likely to coat an active material. However, since the adherence between an active material and a current collector is low, there is a problem such that satisfactory enhancement of cycle characteristic, which is one of the important characteristics of batteries, becomes difficult.
Further, it has also been considered to apply an aqueous dispersion containing acid-modified polyolefin resin to a current collector to obtain an electrode for a secondary battery (PTL 3). In this technique, however, an aqueous dispersion for forming a mixture layer contains an organic solvent. Thus, there has been a problem of a trace amount of the organic solvent being more likely to remain in a mixture layer, making irreversible capacity more likely to be increased. Moreover, from the viewpoint of environmental consideration (elimination of VOC (volatile organic compound)), less organic solvent is required to be contained in the aqueous dispersion.
Furthermore, PTL 4 proposes a binder composed of a neutralized ethylene-(meth)acrylic acid copolymer. In this technique, carboxylic acids contained in the ethylene-(meth)acrylic acid copolymer are neutralized with a volatile neutralizing agent (such as an amine) to prepare an aqueous dispersion of copolymers. Then, the aqueous dispersion is applied together with an active material to produce a mixture layer.